In recent years, delivery of a wide variety of services (data, audio, broadcast, and wireless data) has been expanded by using an access network between an OLT (Optical Line Terminal) and an ONU (Optical Network Unit). With the expansion of such an information service, a PON (Passive Optical Network) system based on a P2MP (Point-to-Multi Point) connection that achieves inexpensive and stable broad-bandwidth even at a location distant from a station facility has been widely used, instead of an xDSL (Digital Subscriber Line) that is a transmission system using a metal line.
In a commercial access network using the PON system, a GE (Gigabit Ethernet®)-PON is currently providing a service, which employs a WDM (Wavelength Division Multiplexing) that enables a bidirectional communication with a single optical-fiber transmission line and applies a TDM (Time Division Multiplexing) to a downstream transmission (OLT (Optical Line Terminal) →ONU (Optical Network Unit)) and a TDMA (Time Division Multiple Access) to an upstream transmission (ONU→OLT) (see, for example, Non Patent Literature 1). In addition, by applying a high-speed TDM technique, a 10G-EPON having a bit rate 10 times higher than the GE-PON has been developed (see, for example, Non Patent Literature 2).
Furthermore, not only an effort to achieve a high-speed commercial access network but also an effort to achieve a low power consumption with respect to the OLT and the ONU has been continued in order to reduce increase of the power consumption and environmental load associated with a high speed and high functionality of the communication device. To this end, a standardization of a power-saving mode in which an effect of reducing the power consumption can be obtained by intermittently controlling a conduction time of each module and unit during a time for which there is no exchange of data between the ONU and the OLT (sleep time control) has been studied by standardization organizations (the ITU-T (International Telecommunication Union Telecommunication Standardization Sector), the IEEE (Institute of Electrical and Electronic Engineers), and the like) (see, for example, Non Patent Literature 3).
Further, in the future, the PON system is highly expected as means for building an inexpensive mobile backhaul to be connected to a mobile base station (BTS: Base Terminal Station) from increase of a traffic by a mobile terminal that is rendered smart, that is, highly functional. This has also brought a discussion on clock synchronization and time synchronization function in the PON system to the standardization organizations.